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First of all, it's safe to assume that the Source engine gets a lot more active development than the engine used in Trine 2, whose name I forget, and Source has definitely been used for many more high-budget games. Of course it's going to be more complicated.

The engine in Trine 2 is probably just as taxing as Source, even if Trine 2 did not have as huge a budget as say Left 4 Dead. It uses a lot of different graphics effects and features as it's graphics were the game's main selling point in comparison to other games in the same genre. Let us not forget that Source has quite a lot of legacy parts to it as well and has not kept up as well as certain other engines (Unreal3, Crytek, id Tech 5) in terms of graphics and effects.

More importantly, anyone who's ever worked on or debugged an OpenGL implementation knows that different applications use different features in different combinations, even if they are visually similar from a user's perspective, and they thus expose different driver bugs.

This is of course true, although you would need to demonstrate this before it becomes anything more than conjecture.

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Just had another idea, could be Valve are holding out to have L4D2 be the 100th Steam for Linux game release. It is up to 88 atm, shouldn't be more than a week or two till it reaches the 100 game milestone.

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The engine in Trine 2 is probably just as taxing as Source, even if Trine 2 did not have as huge a budget as say Left 4 Dead. It uses a lot of different graphics effects and features as it's graphics were the game's main selling point in comparison to other games in the same genre. Let us not forget that Source has quite a lot of legacy parts to it as well and has not kept up as well as certain other engines (Unreal3, Crytek, id Tech 5) in terms of graphics and effects.

It may very well be as taxing - performance isn't what I was referring to. The recency of the features used by an application is one factor affecting its compatibility with different drivers, but by no means the only factor.

More importantly, anyone who's ever worked on or debugged an OpenGL implementation knows that different applications use different features in different combinations, even if they are visually similar from a user's perspective, and they thus expose different driver bugs.

This is of course true, although you would need to demonstrate this before it becomes anything more than conjecture.

Well, I can't exactly demonstrate it for L4D2 specifically when it hasn't been released yet.

If you just want an example for an application, though, I would refer you to my experience with Psychonauts on Mesa with the nv50 Gallium driver. This was a driver that rendered Trine 2 just fine, for instance, but on Psychonauts - a port of a game from last generation, and by no means cutting edge - half of the textures in the game were flickering between displaying solid white and the actual texture. It turned out that one of the few hundred shaders it was using was calculating a square root as the first operation in a basic block, and the nv50 compiler was emitting one instruction for square roots at the start of basic blocks when it should have been emitting two. This is the kind of obscure, application-specific and driver-specific bug that plagues graphics drivers. I gave you one example, but uncovering these sorts of obscure driver issues is quite common. Even more so for a game using the Source engine, which is configurable enough to provide an almost infinite number of cases that could uncover a bug in either the Nvidia, AMD, or Intel driver.

Of course, as you say, it is still conjecture that this is what's actually holding it back. It's just a rather likely one as far as conjectures go.

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Or more likely: They're releasing them at a predetermined pace in order to get the most sales. The first game they released was free, and running on the newest engine. They've also released old games on old engines (HL, CS). I wouldn't be surprised if L4D (1) dropped any day now, with L4D2 two or three of months behind.